Modifying the Xiang Sheng 708B [56K!!!!]

[dcheming]

Originally Posted by zer061zer0

We use some 240v over here

So since your transformer is rated for 220V, but you're feeding it 240V you'll have the same problem with high heater voltage that we have here unfortunately.

[Chops]

Originally Posted by dcheming

So since your transformer is rated for 220V, but you're feeding it 240V you'll have the same problem with high heater voltage that we have here unfortunately.

Do you think it might be a little worse since they're 20V higher where we're only 10V higher? Maybe pushing close to 8V to those heaters?

[zer061zer0]

I dun quite understand how we can connect the resistors in line to the PS and to the heater pins, Derek can you show me another pictures.

[dcheming]

Originally Posted by zer061zer0

I dun quite understand how we can connect the resistors in line to the PS and to the heater pins, Derek can you show me another pictures.

So the yellow twisted pair coming from the transformer is the heater winding for the 6Z4 and 6N1 up front. The green twisted pair coming from the transformer is the heater winding that feeds the three audio tubes.

By cutting one wire in the twisted pair and inserting the resistor there we are basically putting the resistor in series with the the heaters. I would start with the audio tubes first. So what is the voltage of your audio tube heaters?

[dcheming]

Here's how I did the resistors for the audio tubes. I soldered about 6" of new twisted green wire to the audio board to give me more slack to play with. For the rectifier tube I de-soldered one of the yellow wires from the PS PCB, soldered about 2" of green wire in it's place and then soldered it and the yellow wire to the 1.5Ω resistor.





I'll be able to calculate what size you need for this resistor based on what your heater voltage is. Let me know if you want some close up pics of the rectifier area.

[Chops]

I just measured mine, and it's at 7.58V. A little more than your amp, Derek.

So what do you think, maybe a 1.5 ohm resistor?


As for measuring the heater voltage on the 6Z4, can I just place the probes on the resistors' (R3,4) terminals closest to the socket to get a reading?




As usual, thanks!

[dcheming]

If your audio tubes draw around the same current as mine do then a 1.5Ω resistor should drop the 7.58VAC to around 6.4VAC according to calculation and a 1.63Ω would be ideal in your amp.

Yeah measuring the side of those resistors closest to the socket is perfect.

[Chops]

Originally Posted by dcheming

If your audio tubes draw around the same current as mine do then a 1.5Ω resistor should drop the 7.58VAC to around 6.4VAC according to calculation and a 1.63Ω would be ideal in your amp.

Yeah measuring the side of those resistors closest to the socket is perfect.

Cool. So 1.63 ohms it is.

I'l check the 6Z4 heater voltage tomorrow and post the results. I don't want to go probing around in there now. As tired as I am, I might fry the fool thing, or myself! LOL

BTW, I sent you a PM.

Thanks again!

[dcheming]

Try the 1.5Ω first and record what the voltage drops to. Using that we can calculate the ideal resistor for your tubes in your amp instead of using the numbers I got from mine. You'll have to do this for the rectifier anyways since the voltage test data I have is for the 6X4.

[Chops]

Originally Posted by dcheming

Try the 1.5Ω first and record what the voltage drops to. Using that we can calculate the ideal resistor for your tubes in your amp instead of using the numbers I got from mine. You'll have to do this for the rectifier anyways since the voltage test data I have is for the 6X4.

That reminds me... I still need to get a 6X4.

Also, I just measured the 6Z4 heater and it was up at 7.09V. Just like we spoke about last night in the PM, I measured at the resistor terminals closest to the 6Z4 socket.